The incidence of injury to the anterior cruciate ligament (ACL) has been increasing over the past several years. Consequently, ACL replacement surgery using autografts is being performed between 75,000 to 100,000 times per year in the United States alone, and the results have not been consistent in restoring knee function to pre-injury levels. Over 35% of patients have been reported to have complications or unstable knees shortly after reconstruction while long-term follow-up at 5 to 10 years has also revealed that 15-25% of patients have unsatisfactory results. Therefore, research efforts to better understand the forces in ACL replacement grafts during in vivo activities and to compare them with the intact ACL are needed to design better surgical approaches and rehabilitation protocols as well as to improve the long-term outcome. The overall objective of this proposal is to measure the in situ force distribution in the ACL and ACL replacement graft during activities of daily living and rehabilitation exercises. In addition, rehabilitation exercises will be rank-ordered according to the forces in the ACL replacement graft. Such scientific data are needed to resolve the debate between aggressive and conservative post-operative rehabilitation. A novel and newly developed robotic/universal force-moment sensor (UFS) testing system will be employed to determine the force distribution in the ACL and ACL replacement graft in cadaveric knees based on in vivo kinematic data obtained from healthy volunteers and ACL patients. The unique and innovative methods utilized in this proposal should yield quantitative data to identify deficiencies in and improve the results of ACL reconstruction. Also, the data obtained can contribute to the design of appropriate post-operative rehabilitation regimens that are scientifically based. The overall hypothesis is that the outcome of ACL reconstruction will be improved when the ACL graft can reproduce the function of the intact knee, both at immediate post-operative periods and during rehabilitation. Ultimately, the results from the proposed studies could also be used in computational models of the knee such that the function of the ACL and ACL replacement grafts during more complex and dynamic in vivo activities could be studied.